Validating your model with Web API

One of the great things about ASP.NET 4.5’s Web API is that it’s built on the same (or similar) principles as MVC. This means that you get a lot of cool things out of your API controllers from MVC – like Action Filters.

While building my first Web API controller, I wanted to ensure that a creation or an update of an item was only done if that item was valid. I also wanted to pass any validation errors back to the client. This looks quite difficult at first because the Put and Post functions on an ApiController can’t return a result. Action Filters to the rescue!

With a simple action filter attribute, you can ensure that your models are validated and the errors are returned in a simple format to the client by decorating the appropriate methods.

Now a Create (PUT) and an Update (POST) request will validate the model before invoking the appropriate action. Calling Create from using a JSON request with an invalid model would look something like this:

Request

Method: PUT

Body: { Title: ”, IsDone: false }

Response

Status Code: 400 (Bad Request)

Body: [{ “key”: “Title”, “value”: “The Title field is required” }]

Using a valid model will simply result in a 200 (OK) response with no body.

NOTE: while writing this post I stumbled on these two blog posts that did the same thing and adapted my code: